A tensioning device of the foregoing type is shown in FIG. 26 of WO No. 83/00904. This reference discloses a tension roller mounted on a support arm which is swivable against the resistive force of a spiral spring. A hydraulic rotation damper is arranged coaxial to the swivel bearing which, during pulsed forces originating in the movements of the tensioned drive belt, acts in the direction of swiveling. The reference further describes a linearly movable tensioning device with linear hydraulic damping elements and corresponding helical springs. The damping action is constant during operation, for example, of an automobile engine and leads to relatively slow adjustment of the tension force of the drive belt acting against the tension spring or to high-frequency vibration. This known tensioning device using inserted hydraulic dampers offers a satisfactory solution, but the long-term behavior of such dampers, as a result of the almost unavoidable leakage loss is extremely bad particularly for damping the swiveling action produced by vibrations. Therefore the remedy for damping the aforementioned vibrations becomes ineffective very early. Furthermore, this leads to a constantly increasing noise due to load alteration, which in the case of shock absorbers inserted at other places on the vehicle signals the necessity of replacement. Although hydraulic damping, with the damping moment progressively increasing with the speed of movement, is advantageous in the case of shock absorbers for the springs of a vehicle it need not be used in the tensioning device for a drive belt.
Another tensioning device with rotation damper and helical spring is disclosed in DE-OS No. 3225411. The damping is attained in this case by a concentrically arranged plastic sleeve. Small swiveling movements are damped in accordance with this construction by flexure of the plastic material, while large swiveling movements are damped by skin friction. The concentric arrangement inside the spring effects a very small relative movement of the frictional surfaces against each other during swiveling movements of the tensioning device as a consequence of the small diameter of the plastic sleeve. In the case of small swiveling movements, damping results from only the elastic deformation of the material prior to overcoming of the static friction. This is intended in accordance with the reference and produces essentially no damping without extra resilience due to material elasticity.